The present invention is directed to a process for the preparation of aqueous polyurea dispersions having improved stability in water by the emulsion polymerization process and to the aqueous polyurea dispersions prepared by this process.
Aqueous polyurethane dispersions are known and are generally prepared by reacting a polyisocyanate with a high molecular weight polyol and low molecular weight chain extender components including components which provide hydrophilicity to the polyurethane. In these known processes, the polyisocyanate is initially reacted with the high molecular weight polyol to form an NCO prepolymer either in the melt or in the presence of an organic solvent prior to the introduction of water.
In U.S. Pat. No. 3,479,310 the NCO prepolymer is chain extended to form a linear polyurethane which is then mixed with an organic solvent prior to the addition of water. In U.S. Pat. Nos. 4,066,591 and 4,408,008 the NCO prepolymers are mixed with a solvent, dispersed in water and then chain extended with low molecular weight polyamines.
Accordingly, even though the chain extension step may be conducted in either the organic or the aqueous phase, the reaction between the polyisocyanate and the high molecular weight polyol is not conducted in the presence of water in either process. Further, in both processes organic solvents are necessary to reduce the viscosity of the fully reacted polyurethane or the NCO prepolymer prior to dispersing them in water.
An object of copending application, U.S. Ser. No. 07/677,011, filed Mar. 28, 1991 now U.S. Pat. No. 5,212,230, was to provide a method for preparing aqueous polymer dispersions which did not require the use of organic solvents to reduce the viscosity of fully reacted polyurethanes or NCO prepolymers and which could be conducted using readily available, simple mixing equipment. In accordance with this application an aqueous polyurea dispersion was prepared by emulsion polymerization, i.e., a polyisocyanate was dispersed in water and subsequently reacted with a high molecular weight polyamine. In this manner the need for solvents to disperse NCO prepolymer or fully reacted polyurethanes was avoided.
One of the disadvantages of this process is that the polyurea dispersions do not remain stable for more than a few hours. This becomes a problem if the dispersions cannot be prepared at the site where they will be used, which is often the case.
Accordingly, it is an object of the present invention to provide aqueous polyurea dispersions which possess improved storage stability without sacrificing any of the valuable properties provided by the aqueous polyurea dispersions described in the copending application. It should be possible to prepare these dispersions which remain stable in storage until they are ultimately used for their intended purpose.
Surprisingly, this object can be achieved in accordance with the present invention as set forth hereinafter.